This invention relates to a method for coating and rapidly curing a mercaptoalkyl-containing polydiorganosiloxane on a solid substrate. In one aspect this invention relates to a method of coating cellulosic paper with a silicone release coating to provide premium release of adhesive materials therefrom.
In the pressure sensitive adhesive art it is often desirable to prepare an adhesive-bearing article such as a label or a tape and to store the article for some time before use. In the storage process the adhesive-bearing article is conveniently adhered to an intermediate surface from which it must be removed before use. For example, said tape is wound in a coil whereby the adhesive surface of one turn of the tape is adhered to the non-adhesive surface of the previous turn of the tape. Preferential release of the upper turn of tape with its adhesive from the previous turn is needed to unwind and use the tape. In another example, adhesive-bearing labels are frequently supplied with a protective paper over the adhesive, which paper must be easily removable from the adhesive before the label is applied in its intended use.
Polyorganosiloxanes have been used for a long time to coat solid substrates, such as paper, to improve the release of adhesive materials therefrom. As the adhesive properties and holding power of adhesive materials have improved, the need for improved release coatings has increased. Improved release coatings have also been required by other factors such as increasing environmental concerns, a desire for faster coating and curing processes, and a need to lower process energy costs.
Solventless, radiation-curable coatings have thus been sought that would provide improved release and preferably premium release of adhesives, particularly aggressive acrylic adhesives. A coating is considered to provide premium release if a force of not more than 100 grams per inch (38.61 newtons per meter), as measured by the method hereinafter described, is needed to remove the adhesive from the coating. Of course, the release force should not be so low as to allow the adhesive to fall off or to be accidentally displaced from the surface of the substrate. A release force of at least 20 grams per inch is usually sufficient for this purpose. An aggressive adhesive is a material that requires a force of at least approximately 460 newtons/meter (N/m) to remove the adhesive from a stainless steel mirror surface using said method of measuring.
Radiation-curable coatings comprising mercaptoalkylsubstituted organopolysiloxanes are taught by Berger, et al., U.S. Pat. No. 3,726,710; Michael, et al., U.S. Pat. No. 3,873,499; Gant, British Pat. No. 1,409,223 and Bokerman, et al., U.S. Pat. No. 4,052,529. However, these disclosures teach that an organosilicon compound containing vinylic unsaturation is needed to cure the mercaptoalkyl-substituted organopolysiloxane. The resulting compositions are thus multi-component compositions and are considered less desirable, from cost, preparative and stability viewpoints, than one-component compositions.
Radiation curing of organopolysiloxanes, including ultraviolet light curing of organopolysiloxanes containing functional organic groups such as organic groups containing hydroxyl, amino, carboxyl, ester, carbamyl, nitrile, amide, hydrosulfide, sulfate, nitro and sulfide substituents has also been discussed by Warrick in Canadian Pat. No. 653,301. However, Warrick's broad disclosure relating to organopolysiloxane structure and reciting long cure times, ranging from a few minutes to several hours, offers no solution for the problem of coating a substrate with a one-component, radiation-curable coating composition that will cure in seconds and will provide premium release of adhesives.
One-component mercaptoalkyl-containing organopolysiloxane compositions have been applied to vinylic polymers by Gowdy, et al., U.S. Pat. No. 3,535,145; to copper and silver by Viventi, U.S. Pat. No. 3,346,405 and to fuser elements in electrostatic copiers by Imperial, et al. U.S. Pat. No. 4,029,827. The disclosures of Viventi and Imperial, et al. relate to applying a mercaptoalkyl-containing organopolysiloxane to a substrate whereupon a reaction of some sort, other than curing, occurs between the substrate and the applied mercaptoalkyl-containing organopolysiloxane. However these disclosures do not relate to curing one-component coatings to provide release of adhesives. Gowdy, et al. further apply heat or actinic radiation to the surface of the vinylic polymer to initiate a reaction that irreversibly bonds the applied organopolysiloxane thereto. However, Gowdy, et al. fail to provide a method for curing a mercaptoalkyl-containing polydiorganosiloxane on a solid substrate, such as paper, in seconds to provide premium release of aggressive adhesives.